I have the exact same issue. Its because the Adaptive Sync. When it's getting below the freesync range of 55-144fps the black screen flickering starts. I have contacted Acer and they want me to sent it back. They said they haven't heard before. Please contact ACER support team. This will push for a new firmware realise I hope. Looks like they haven't tested this well on a AMD card.
I tried many many things. It's definitely the Monitor in combination with a amd card. Also tried it with a Vega card. Same issue!!
I do see similar issues with Enhanced Sync on amd release notes / known issues but those aren't fixed for a long time. So don't trust this will be fixed from AMD side soon.
Samsung did had similar issues and they fixed this with a monitor firmware update.
